In a conventional electrically operated melting furnace electrodes project into a furnace tank and are immersed in molten material disposed in the tank with batch material continually being supplied on top of the molten material to provide both a source of supply and an insulating layer or crust. Additional batch material may be supplied, as needed, by a feeder, typified by U.S. Pat. Nos. 3,980,460 and 4,142,880. These batch materials, when spread in a layer over the upper melt surface or melt line, serve as a partial insulating blanket to minimize heat loss to the surrounding environment. Problems arise when a uniform layer of batch material of a certain minimal thickness is not maintained over the whole upper surface of the melt line. Regions of the batch layer that are too thin "melt out", expose the molten material and create regions of excessive heat loss with a concomitant lowering of the temperatures of the molten material or melt and a lowering of the melt flow rate. In addition, regions of thinly covered melt may allow equipment located over the furnace, such as a batch feeder, to be exposed to excessive heat. When too thick a layer is deposited, the melt temperature may rise and uncontrollably increase the melt flow rate. Too much batch material also promotes formation of a surface crust which hinders the passage of the overlaying batch material to the melt line.
Inability to maintain a uniform thin layer of batch at an optimal thickness contributes to undesirable hot and cold thermal cycling of the melt with a resultant undesirable fluctuation of furnace performance. These ill effects are aggravated when the operating temperature of the furnace is increased. In refractory furnaces, which typically operate at 3400.degree. to 4000.degree. F., the need is greatest for a controlled deposit of batch material.
The batch feeders disclosed in the prior art do not insure that a thin, uniform layer of batch material is deposited over the melt. Another typical disadvantage is that the feeders require high maintenance. An ideal batch feeder would provide a controlled and uniform batch covering, generate little or no dust conditions in the working environment proximate to the furnace, be mechanically simple, inexpensive to construct, operate and maintain and would be highly reliable.